Green (uncured) elastomers do not have the requisite strength necessary for the many desirable uses ranging from hoses and cable coatings to tires and insulating materials. Several cure systems are known and used in practice to crosslink various elastomers and to produce compositions having superior properties. More specifically, various cure systems are described for polyisobutylene based elastomers, see for example Rubber Technology, M. Morton, pp.294-314, Van Nostrand Reinhold ed. New York 1987. Polyisobutylene based elastomers containing a minor portion of a conjugated diolefin known as butyl rubber, can be cured by three basic types of cure systems, by sulphur and organic accelerators such as thiurams or dithiocarbamates, by dioxime and related dinitroso compounds with lead oxide and/or a sulphur donor, or by phenol formaldehyde resins containing reactive methylene groups with or without additional activators. Zinc oxide and stearic acid are also usually added. Halogenated isobutylene copolymers can be cured by zinc oxide, usually accelerated by thiurams and thioureas, by zinc oxide and a bifunctional dienophile such as bismaleimide such as disclosed in U.S. Pat. No. 5,200,469, by diamines such as hexamethylene diamine carbamate, or by reactive phenolic resins and zinc oxide. Essentially all these cure systems also require an acid scavenger, the acid generated during curing reaction being either hydrogen sulfide in the case of sulphur cure, or hydrochloric- or hydrobromic acid in the case of halogenated polymers.
Likewise, low molecular weight polyolefin or polyisoolefin based elastomers containing a low level of chemically bound functionality such as an hydroxyl or an amine can be conveniently blended with and cured by an unblocked or blocked polyisocyanate. For example, U.S. Pat. No. 4,939,184 discloses the preparation of flexible polyurethane foams made by reacting a low molecular weight polyisobutylene having two or three terminal hydroxy groups with a polyisocyanate in the presence of a blowing agent. The molecular weight of polyisobutylene is limited to within the range of 1,000 to 10,000, preferably within the range of 1,500 to 7,000. When molecular weight exceeds 10,000 viscosity becomes extremely high and blowing becomes difficult. Urethane compositions based on low molecular weight polyisobutylene polyol containing terminal hydroxyl groups and reacted with an organic polyisocyanate compound are also disclosed in JP.2-202909. The polyisobutylene polyol described as suitable in JP.2-202909 has a number average molecular weight M.sub.n between 700 and 8,000, and preferably between 1,000 and 5,000. Unfortunately, however, when M.sub.n of the polymer is increased to above 8000, the composition becomes highly viscous, fluidity decreases, complete injection becomes impossible during casting, impregnation and coating processes and insulation defects result. These difficulties with higher molecular weight polymers result in part from using polyisocyanate containing free isocyanate groups as curing agent. Higher M.sub.n polymers are typically heated to make them amenable to the blending process. The polyisocyanate will cure the polymer at the higher temperatures needed to soften the polymer to make it amenable to blending. Thus polyisocyanates containing free isocyanate groups are not traditionally used to cure higher M.sub.n polymers. Note that JP 2-202909 states that their compositions are cured by heating to between 90 and 170.degree. C., but are blended at lower temperatures. Higher M.sub.n polymers are generally heated to these higher temperatures (90.degree. C. to 170.degree. C.) to permit blending to occur at all. Thus a polymer/polyisocyanate mixture heated to between 90 and 170.degree. C. would crosslink prematurely during blending, could not be processed into the desired shape, making the blend unsuitable for any practical use. Hence polyisocyanates are not generally considered appropriate for curing high M.sub.n polymers.
Diisocyanate containing free isocyanate groups has also been used to cure copolymers of isobutylene and modified styrene containing tertiary aminoalcohol groups in EPA 325 997. EPA 325 997 discloses diisocyanate curing of polymers having a molecular weight of 700 to 200,000, however only exemplifies blends of up to about 30,000 weight average molecular weight M.sub.w and about 8,600 M.sub.n, as measured by gel permeation chromatography. Further the examples are blended in solution at room temperature. The solution gels in a few minutes. Mixing conventional unblocked polyisocyanate with high molecular weight containing hydroxyl or amine functionality is unpractical and leads to premature crosslinking during mixing even at moderate temperature. In contrast the invention herein provides a way to safely mix, mold, and cure high molecular weight elastomer compositions with polyisocyanates without causing the undesirable side effects. In particular this invention describes the use of a blocked polyisocyanate that allows safe mixing and incorporation of the compound at temperature as high as 120 to 150.degree. C. without inducing undesirable crosslinking during mixing.